The gas-phase fluidized bed reactors can be used to produce a variety of polyolefins. The olefin polymers produced by fluidized bed reactor are usually in the form of powder, and fluidization is supplied by a circulation gas consisting of one or more monomers, an inert gas and the like. The conventional gas-phase fluidized bed polymerization apparatus consists of a fluidized bed reactor, a circulation gas compressor, and a circulation gas cooler to form a closed circulation loop, and a circulation gas consisting of reaction monomer ethylene, α-olefin, hydrogen gas and inert gas nitrogen is operated in the circulation loop and brings out the heat released from the polymerization reaction. The catalyst used in the gas-phase fluidized bed polymerization reaction process includes a Ziegler-Natta catalyst, a chromium oxide catalyst and a metallocene catalyst. Because the traditional gas-phase fluidized bed polymerization reaction process has the characteristics of less equipment, short process flow, flexible operation, less discharge, no solvent and low noise, it is an economical and environment-friendly petrochemical industry technology, and gradually becomes the mainstream of the polyethylene process.
For polyolefins, especially polyethylene, the molecular weight and the molecular weight distribution greatly affect the mechanical properties and processabilities of the polymers. In the study of the art, it has been recognized that the higher the molecular weight, the higher the mechanical properties. However, the high-molecular-weight polyolefins have poor fluidity, are difficult to process, and are difficult to blow and extrude at a high shear rate. It is known in the art that expanding the molecular weight distribution of polyethylene can achieve the purpose of improving the rheological properties while maintaining the mechanical properties of the final product; wherein the high-molecular-weight portion ensures the mechanical properties of the product, and the low-molecular-weight portion contributes to improving the processabilities of the product.
A single conventional gas-phase reactor can only produce polyethylene products with single distribution. It is known in the art that simple melt blending of two ethylene polymers with high and low molecular weight distributions can result in heterogeneity of the product. In addition, it has been recognized in the art that it is possible to produce polyethylene having broad/bimodal distribution by subjecting the catalyst or the polymer having the active site to two or more different reaction conditions or gas compositions for reacting continuously.
In the past, bimodal polyethylene is produced in such a way that two or more reactors under different reaction conditions are used in series. European Patent EP-A-691353 describes a process for producing broad/bimodal polyethylene by two conventional gas-phase reactors connected in series; in which the problems that the reactants are routed with each other in the two gas-phase reactors, the residence time of polymer particles in the two gas-phase reactors is not equal and the like are caused. Patent EP-B-517868, U.S. Pat. No. 6,642,323 and U.S. Pat. No. 7,115,687B disclose a process in which a first loop reactor and a second gas-phase fluidized bed reactor are connected in series; in which the problems that the residence time of polymer particles in the two gas-phase reactors is not equal, and the resin produced in the first reactor contains more fine powder are caused.
European Patent EP-B-1012195 and Chinese Patent Application No. 2004800305663 propose a multi-area circulation reactor and process for producing polyolefins having a bimodal molecular weight distribution. The reactor is characterized by comprising an ascending section in a rapid fluidization state and a descending section in a moving bed state which are connected with each other. The two patents have the problems that the reaction gas is routed, and the descending section in the moving bed state cannot be applied to polyethylene production with large heat release. Chinese Patent Application CN 102060943 A discloses a process for preparation of bimodal polyethylene and a gas-phase reactor comprising at least four fluidized beds. In the process, the polymer particles are bubble-fluidized in the first reactor or the third reactor, and the fine powder in the polymer particles is entrained and blown by the elation to the second reactor or the fourth reactor in a rapid fluidization state, the polymer fine powder enters a cyclone separator from the outlet of the second reactor or the fourth reactor for gas-solid separation, and the circulation gas is returned to the reactor in the same reaction condition via the circulation loop; and the solid polymer particles enter another bubbling bed in different reaction conditions. The process has the problems that the operation is complicated and the circulation amount of the polymer particles between reactors having different reaction conditions is limited by the entrained amount of the fine powder.